Plant-compositional effects on surface runoff and sediment yield in subalpine grassland

Soil erosion such as sheet erosion is frequently encountered in subalpine grassland in the Urseren Valley (Swiss Central Alps). Erosion damages have increased enormously in this region during the last 50 y, most likely due to changes in land-use practices and due to the impact of climatic changes. In order to estimate the effect of vegetation characteristics on surface runoff and sediment loss, we irrigated 22 pasture plots of 1 m2 during 1 h at an intense rain rate of 50 mm h-1 in two field campaigns using a portable rain simulator. The rain-simulation plots differed in plant composition (herb versus grass dominance) and land-use intensity but not in plant cover (<90%) nor in soil conditions. Prior to the second rain-simulation campaign, aboveground vegetation was clipped in order to simulate intense grazing. The generated surface runoffs, sediment loss, relative water retention in the aboveground vegetation, and changes in soil moisture were quantified. Runoff coefficient varied between 0.1% and 25%, and sediment loss ranged between 0 and 0.053 g m-2. Thus, high infiltration rates and full vegetation cover resulted in very low erosion rates even under such extreme rain events. Surface runoff did not differ significantly between herb- and grass-dominated plots. However, clipping had a notable effect on surface runoff in the test plots under different land-use intensity. In plots without or with intensive use, surface runoff decreased after clipping whereas in extensively used plots, surface runoff increased after the clipping. This opposite effect was mainly explained by higher necromass and litter presence at the extensively used plots after the clipping treatment. The results obtained here contribute to a better understanding of the importance of vegetation characteristics on surface-runoff formation, thus, on soil-erosion control. Overall, we delineate vegetation parameters to be crucial in soil-erosion control which are directly modified by the land-use